Test circuits are known in which a series resistor is connected in a load circuit and the voltage drop across the resistor serves as a measure for checking the continuity of the load circuit. See published German patent application DE-OS No. 2,147,681. Such a voltage drop is generally undesirable in the circuit. If the voltage drop is kept at a low value by use of a low resistance, the detection circuit for sensing the voltage drop to indicate continuity is of increased complexity. Further, testing of the load circuit continuity is possible only with the load circuit switch on in order to produce the voltage drop. This is sometimes referred to as hot interrogation or hot testing. If the load circuit is switched off, a discontinuity in the load circuit is not detectable with such a hot test circuit.
In the published German patent application DE-AS No. 2,363,091 it is proposed to test the continuity of the load circuit before the load circuit is turned on. This sometimes is referred to as cold interrogation or cold testing. In this prior art, a special circuit arrangement is adapted to detect the defective stop light of a vehicle before the brake is applied to energize the brake light. However, this circuit arrangement is applicable only to two loads connected in parallel, as is the case with stop lights of a vehicle. This circuit only indicates failures which exist before the first brake actuation.
In the published German patent application DE-OS No. 2,906,878, a reed switch is disclosed for controlling a pilot lamp. With this circuit arrangement only hot testing is possible.